internal combustion engines - rutgers universityeden.rutgers.edu/~mtenorio/media/ice_project.pdf ·...

Internal Combustion

Engines

Design Project I

Suhas Somnath 650:461

4- 11-2008 Department of Mechanical

and Aerospace Engineering, Rutgers University,

Piscataway, NJ 08854

650:461 Internal Combustion Engines – Design Project April, 10, 2008

Objective:

The main objective of the experiments was to understand and observe the effects of changing different engine parameters that might affect its performance. Maximizing the maximum power output of the engine was chosen to be the main objective. Introduction:

Mass produced engines for vehicles are optimized to meet three main goals – • Keep the price of the vehicle within

a certain range. • Conform to emission, NVH

standards / regulations. • Provide maximum reliability and

longetivity of the engine.2 These very compromises leave a

lot of room for achieving particular goals such as achieving maximum power. Modifying an engine is a delicate art. Carelessness while doing so may have a detrimental effect on the engine and its auxiliary components. One must be prudent enough to foresee if the removal, addition or the modification of a component may make the engine less durable or unstable. Also, modification should be done within the boundary of economics. If the changes made to the engine were too expensive either due to the cost of the parts or difficulty in repairing, such modifications may not be viable when it comes to mass production engines. Theory & Analysis:

Changing different components of an engine have different effects on its performance, durability, emissions,

lifespan, etc. Care must be taken to make sure that adverse effects of any such changes are minimized or reasonable compromised with the improvement in performance. One must be vigilant of the cost involved in offsetting the side-effects of these modifications.

Valves: Most engines have significant room for improvement in the valves and cylinder head. The intention is to maximize the breathing of the engine by utilizing maximum possible area for the intake and exhaust valves. This increase in cumulative area for intake and exhaust valves can achieved by a combination of increasing the number of intake and exhaust valves and also varying the valve diameters. By increasing the number of valves, valve float is deferred and the engine can rev at higher speeds due to the decreased weight and inertia of each valve. By allowing staggered opening of valves, the gas motion / swirling within the cylinder significantly improves, hence enabling better mixing of the air and fuel mixture and more efficient combustion. There are however, some constraints that must be kept in mind, namely the compromise in the structural rigidity of the cylinder head due to increased vibrations and, stresses and also, the increase in the complexity of the camshaft in attempt to house more cams.

Bore & Stroke: The breathing of the engine can be further improved by increasing the bore which further increases the available area for larger valves. A large portion of the heat introduced into the cylinder during the combustion stroke and the heat generated during the power stroke is lost to the cylinder walls. As a result a significant portion of the fuel is left uncombusted. By calculus it can be proven that for a cylinder of constant

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volume, surface area is minimized when the following condition is met:

To be on the safe side, the bore to stroke ratio must lie within the optimal prescribed range (0.8 – 1.2 for a small SI engine)1. Enlarging the bore by a large amount results in an increase in the clearance volume and hence a decrease volumetric efficiency and also decrease the duration of each stroke. More importantly, the structural rigidity of the engine block can be seriously compromised because of this change. Compression Ratio: To compromise for the increased clearance volume created by the increase in the bore, the compression ratio can be increased. The increase in compression ratio not only decreases the clearance volume but also increases the pressure in the cylinder. Since the pressure is increased, so is the resulting PV work from the power/ expansion stroke. This increase in compression ratio must again remain within the prescribed limits (8 to 12 for small SI engines)1. An attempt to increase compression ratio beyond this range would result in self-combustion of the fuel and might force the need for higher octane fuel. Yet again, a careless increase could result in damage to the engine block, head and other components due to the coupled effect of the increased bore and compression ratio. Rod Ratio: As the rod length increases, the duration for which the piston remains stationary at the top and bottom dead centers increases. Since the fresh charge is compressed for a longer period of time during ignition, the combustion of fuel is more complete. This increased combustion increases the cylinder pressure and hence the power

output of the engine. Again, care must be taken to keep the rod ratio within the prescribed limits (3 – 4)1 to avoid imbalance and instability of the engine. Cam Optimization: The cylinder suffers significant valve overlap between the opening of the intake valve and the closing of the exhaust valve between the exhaust and the intake strokes. During this time period exhaust gases flow into the intake port if the pressure in the cylinder is higher than the intake manifold and fresh uncombusted fuel air mixture escapes into the exhaust. Because of this effect, fresh fuel is wasted and lesser fuel is combusted every cycle. Optimization in the operation of the cams is hence required to correct this problem. By allowing the exhaust valve to open and close early pumping work during the exhaust stroke is minimized. However, by keeping the intake valve open for a longer period of time, one can compensate for the fresh fuel lost during the scavenging by allowing more fresh charge to enter the cylinder. If the exhaust valve is kept open for too long, it might however have other adverse effects such as decreased fuel efficiency and/ or increased emissions. Intake: During the intake stroke, lesser amount of fresh air enters due to friction and air resistance at the intake manifold and / or the air filter. The use of polished intake manifolds, larger air filters and reduced intake piping allow freer movement of air. This increases probability for a fuel molecule to find more oxygen molecules to get combusted. Hence, such a modification increases the amount of combustion and hence the power developed.

Exhaust: The beauty of the exhaust stroke is that the nearly all the combusted product is expelled out of the

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cylinders due to the lower pressure in the exhaust manifold when compared to the cylinders. In practice however, a significant portion of the exhaust stays back in the cylinders because the pressure in the exhaust isn’t much lower than that of the cylinder. The backpressure in the exhaust is also responsible for backflow of exhaust into the cylinder and / or the intake manifold during the valve overlap. This happens due to constriction in the exhaust pipes because of which the exhaust gases accumulate and the pressure in the exhaust pipes / manifold is greater than the atmospheric pressure. Replacing the exhaust manifold with headers corrects this problem to a large extent. One must not forget to optimize the size of the headers as the improvement in the performance comes with a significant increase in the noise from the exhaust. Forced Induction: The result of addition of forced induction devices is similar to that produced by modifications on the intake manifold. They force air to enter the engine at a pressure much higher than atmospheric pressure. Again, because of increased amount of fuel-air mixture, the resulting combustion provides a large boost to the power output. There are many variations in such devices, including centrifugal or positive displacement superchargers, and turbochargers. Each device provides improvement in performance in different power ranges in different amounts. Such devices would not need any changes to the intake manifold and turbochargers don’t need any alterations to the exhaust manifold as they run on the high pressure from the exhaust. As a result, turbochargers are bound to decrease performance of the engine due to the significantly increased backpressure in the exhaust during part load (turbo-lag).

However, at full load, forced induction devices improve performance of an engine significantly. Apparatus: Pro-racing Dynosim engine simulating software. Experiment:

The engine chosen for the

experiment was a 1993 Honda 1.6 liter engine (figure A1, table 1). The objective of the experiment was to use the blue-print of this stock engine to create a new mass production engine which is on par with other contemporary mass production engines of similar displacement volume while approximately conforming to the three constraints – cost, reliability and emissions. Hence, resorting to extreme measures and ridiculous alterations was out of question. The target was to meet the standards of a contemporary sporty hatchback such as the 2008 Volkswagen Polo GTi. Table 1: Stock

Honda Volkswagen Polo GTi

Displacement volume

1.6 Li 1.8 Li

Peak horsepower

73 bhp @ 4500 rpm

150 bhp @ 5800 rpm

Peak Torque 93 lb-ft @ 3500 rpm

148 lb-ft @ 5000rpm

Engine configuration

Inline 4 Inline-4 turbocharged

Throughout the course of the experiment, many configurations were tried for the parameter under change. For

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each new configuration, the costs of the marginal increase in performance and the side-effects caused were weighed and the configuration with the best compromise was propagated. Valves & Cylinder Head: Initially the engine had only 2 valves per cylinder in which case a large portion of the cylinder head was not utilized for improving engine breathing. Hence, the first change done was to increase the number of valves to 4 and 6.

Table 2: Valves

per cylinder

Intake valve

diameter (inch)

Exhaust valve

diameter (inch)

Maximum Power

developed (bhp)

2 1.2 1 73 4 .95 .86 80 6 .75 .65 92

Table 2 (and appendix figure

A2) show that the new valve designs proved to be highly beneficial in improving the performance of the engine. Because of the increased work being done per cycle, the BMEP increased significantly along with the volumetric efficiency. However, in an attempt to maintain the structural rigidity of the cylinder head, the clearance spacing between valves and between the cylinder edges and the valves were maintained as faithful to the original design as possible (Figure A4). Due to the former constraint, the total cross section of the intake and exhaust valves decreased for the 6 valve configuration when compared to the 4 valve model. Economically, the 6 valve model would not have been viable either because of the increased cost of the extra parts and the crowding of cams on the camshaft. Hence, the 6 valve design was ruled out.

The 4 valve design was adopted as it was economically viable for the large improvement in performance it provided.

Bore and Stroke:

The next change applied to the engine was incrementing the bore and decrementing the stroke in order to keep the cylinder volume constant. Statistics (table 3) indicated that a minor increase of 5 bhp or 2.5 lb-ft of torque was found on increasing the bore by 0.1000 inch each time. The bore was enlarged from 2.953 inches to 3.2 inches in an effort to keep the new bore to stroke ratio of 1.06 within the optimal range for this ratio1. The increased room in the cylinder head was put to good use be increasing the size of the valves which further improved the performance. If this configuration were to form the blue-print of a new production engine, this modification could be achieved at no extra cost. If existing engine blocks were to be bored, a non-trivial cost would be incurred. The graph (figure A3) shows that the overall torque increased and the power developed for any given rpm increased mildly. Table 3:

Bore (inches)

Stroke (inches)

Peak horse-power

Current 2.953 3.543 94 bhp Config 1 3.100 3.216 100 bhp Config 2 3.200 3.0183 108 bhp Config 3 3.300 2.8382 115 bhp Compression Ratio:

Now that the clearance volume was increased significantly due to the increased bore, the compression ratio was raised to offset it. Since the SI engines today operate at compression

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ratios close to 9, the compression ratio was increased from the initial 7 to 9. As expected (table 4 ,figure A5), the increased compression ratio significantly improved the power and torque developed. Since this alteration only involved changing the topmost position of the pistons, this safe alteration as well wouldn’t be a significant economic burden. Table 4:

Compression Ratio

Peak Power (bhp)

Peak Torque (lb-ft)

Current 7.00 108 105 Config 1 8.00 117 111 Config 2 9.00 126 119 Config 3 10.00 138 127 Rod-Ratio:

Another small change was the Incrementation of the rod-ratio to an optimal value of 3 from the initial 1.5. This provided a 5% improvement (figure A6) in the engine’s performance. Again, the ratio was kept to the prescribed range to avoid bending or disfiguration of the connecting rod. Connection rods are available in multiple sizes and configurations and hence finding a match for this requirement wouldn’t have been much of a problem. Cam Optimization :

As per the theoretical plan, the exhaust valve was made to open and close early but the intake valve was made to stay open for a longer portion of time. The valve overlap still remained the same, however, due to the increase in the duration at which the intake valve was open, more fresh charge was able to enter the enter the cylinder for combustion. This provided a 5% improvement (figure A7) in the engine’s

performance. Like the previous modification, this alteration too could be achieved at a small or no extra cost. Exhaust:

In an attempt to improve the engine’s exhalation, the stock exhaust manifolds were replaced with small tube headers. The data (table 5) shows a significant improvement in the engine’s performance. Small tube headers were chosen due to their relatively low cost and more importantly the lower exhaust noise (figure A8).

Table 5: Peak

Power (bhp)

Peak Torque (lb-ft)

Stock Intake & Exhaust

126 119

Small Tube Headers 146 139 Large Tube Headers 148 140 Race Headers 155 147

Intake:

To improve the inhalation for the engine, the stock single plane intake manifold was replaced with multiple less restrictive induction systems (table 6). In an effort to keep costs low, we settled with the Dual-plane intake system as it provided a sufficient boost in the engine’s performance at a minimal cost (figure A9). By now, the experimental Honda engine developed sufficient power and torque to meet the target VW Polo despite the smaller displacement volume. Table 6: Peak

Power (bhp)

Peak Torque (lb-ft)

Stock Intake with Small Tube Headers

146 139

Dual Plane Intake 154 141

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Tunnel Ram Intake 162 150 Other Improvements: Forced Induction:

Other changes that could not be accommodated in the project due to limitations of the software used include:

Forced Inductions have a reputation for providing a very large benefit for a power deficit engine. However, to use one, stock intake and exhaust manifolds had to be used. Table 7 shows some of the such devices used.

• Using lighter metals to make many of the parts.

• Using an aluminum engine block instead of cast iron block to dissipate heat faster.

Table 7: Maxi

mum Power

Maximum Torque

Naturally Aspirated, with Headers & Dual plane intake manifold

154 bhp

141 lbft

Roots B&M 250 Powercharger

147 bhp

149 lbft

Rajay 300B Turbocharger

130 bhp

135 lbft

Paxton Centrifugal supercharger

155 bhp

154 lbft

• Tweaking of the ECU. Conclusion:

All main engine parameters were experimented with and an optimal solution was reached to meet the goal. The stock engine was successfully altered to develop lot more power and torque while keeping investments to a minimum. The performance of the experimental engine in the end was good enough to meet the target (VW Polo GTi).

The Paxton centrifugal supercharger could’ve been used due to its massive power benefit over the entire range; however, due to its high cost when compared to other modifications made, it wasn’t economically feasible to use it (almost 10 times the cost of the intake / exhaust systems). However, if one were modifying an existing physical stock engine, adding the supercharger would work out more economical when compared to increasing the bore or modifying the cylinder head (Figure A10).

References: [1] J.B.Heywood, “Internal Combustion Engines Fundamentals”, 1st ed, McGraw Hill, 2003. [2] " What's the best way to increase the horsepower of my car’s engine?" 06 June 2000. HowStuffWorks.com. <http://auto.howstuffworks.com/question395.htm> 09 April 2008.

Appendix:

The graphs below show three sets of curves – one for the stock engine, next, the engine’s performance on the last improvement made and the third depicting the improvement of engine performance due to current changes done to the engine. By inspecting each of the three curves, one can visualize both the total and marginal increase in performance due to each of the changes made to the engine.

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Figure A1: Stock Engine Configuration:

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Figure A2:

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Figure A3:

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Table AT1: Changes to cylinder head, bore and stoke. Bore

(inches) Stroke (inches)

Intake valve diameter (inches)

Exhaust valve diameter (inches)

Stock (2-valve) 2.943 3.543 1.200 1.000 4 valve unchanged bore

2.943 3.543 0.950 0.850

4 valve with enlarged bore

3.200 3.018 1.100 1.000

Figure A4: Cylinder head configuration for different number of valves and bore diameters:

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Figure A5: Compression ratio increased from 7 to 9:

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Figure A6: Increased rod ratio from 1.522 to 3

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Figure A7:

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Figure A8: Improved exhaust:

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Figure A9:

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Figure A10: Graph shows stock engine (plain curves), altered engine with stock intake and exhausts (diamond points), altered engine with small tube header and mufflers and dual-plane intake (square points) , engine with Paxton centrifugal supercharger (triangle points):

Figure A11: Comparison between Stock Engine and Altered Engine

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Figure A12:

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